Electrical and photovoltaic properties through a large multicrystalline Si ingot

Santo Martinuzzi; Isabelle Périchaud; Olivier Palais; Damien Barakel; Michel Gaulthier

doi:10.1117/12.733827

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3 October 2007 Electrical and photovoltaic properties through a large multicrystalline Si ingot

Santo Martinuzzi, Isabelle Périchaud, Olivier Palais, Damien Barakel, Michel Gaulthier

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Proceedings Volume 6651, Photovoltaic Cell and Module Technologies; 66510G (2007) https://doi.org/10.1117/12.733827
Event: Solar Energy + Applications, 2007, San Diego, California, United States

Abstract

Large multicrystalline cast silicon ingots (>310 kg) are cost effective in the photovoltaic industry and attenuate the feedstock shortage. The bulk lifetime τ_n and diffusion length L_n of minority carriers vary through the height due to the segregation of metallic impurities during the directional solidification. The native impurity concentrations increase from the bottom to the top of the ingot, which is solidified last, while the ingot bottom, which is solidified first, is contaminated by the contact with the crucible. It was found that τ_n and L_n are the smallest in the top and in the bottom of the ingot. In solar cells, the evolution is similar, however in the central part of the ingot L_n is strongly increased due to the in-diffusion of hydrogen from the SiN-H antireflection coating layer. The variations along the ingot height of the conversion efficiency η and of τ_n in raw wafers are well correlated, that can predict the values of η, allowing an in-line sorting of the wafers, before solar cells are made. If τ_n is smaller than 1 μs, as observed at the extremities of the ingot, η will be limited to 10% only; if τ_n is higher than 2.5 μs η achieve 15 % at least. In addition, impurity segregation phenomena around grain boundaries are observed at the extremities of the ingots, linked to the long duration of the solidification process. Reducing the height of the ingots could suppress these phenomena and not much material must be discarded. Another problem can come from the use of upgraded metallurgical silicon feedstock in which the densities of boron and phosphorus are very close. Due to the difference in the segregation coefficients, ingots may be entirely or partly p or n type, suggesting that a purification step tawards the dopants is required.

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Santo Martinuzzi, Isabelle Périchaud, Olivier Palais, Damien Barakel, and Michel Gaulthier "Electrical and photovoltaic properties through a large multicrystalline Si ingot", Proc. SPIE 6651, Photovoltaic Cell and Module Technologies, 66510G (3 October 2007); https://doi.org/10.1117/12.733827

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